The effect of dietary protein deficiency and protein concentration of the diet on the pancreatic trophic response to a CCK analogue (cerulein) were studied. Rats were fed for 14 days with semipurified diets containing 5, 30, or 60% casein. During the final 4 days, they received 2 micrograms/kg cerulein or gelatin vehicle subcutaneously three times/day, and the effects on pancreatic weight and pancreatic content of protein, RNA, DNA, amylase, and chymotrypsin were determined. Cerulein failed to increase significantly any pancreatic parameter in rats fed 5% casein, while stimulating significant increases in almost all parameters in rats fed 30 and 60% casein diets. In the absence of cerulein treatment, increases in dietary protein levels caused progressive increases in all pancreatic growth parameters with the exception of amylase. In the presence of cerulein, increases in dietary protein concentrations caused progressive increases in pancreatic growth parameters (except amylase), which were maximal at 30% casein concentration of the diet for most parameters. The results confirm that pancreatic growth is stimulated by increasing protein concentration of the diet and indicate that a low protein diet, acting through a deficiency of dietary nitrogen and essential amino acids, limits the pancreatic trophic response to CCK or analogues. These results explain the failure of trypsin inhibitors to stimulate pancreatic growth in rats fed low levels of dietary protein.